1. Field of Invention
This invention relates to a valve positioner using digital communication; and more particularly, to an improvement thereof, wherein the current to be allocated to a current-to-pneumatic conversion module can be increased; and wherein, the invention can be applied to convert electrical signals to pneumatic signals.
2. Description of the Prior Art
A valve positioner directly controls the opening of a valve and its feedback signal uses a valve opening signal or a stem position signal. A current-to-pneumatic converter converts an electrical signal, such as, for example, 4 to 20 mA, into a pneumatic signal such as 0.2 to 1.0 [kgf/cm2]. An example of a prior valve positioner is disclosed in Japan Unexamined application 9/144,703.
FIG. 1 shows a conventional valve positioner 100, wherein an operating signal for valve positioner 100, using an electrical signal, such as for example, 4 to 20 mA, is inputted to terminals T1 and T2. Variable impedance circuit 3 and shunt regulator 4, connected in series, are connected to input terminals T1 and T2. Internal power voltage V2, which drives the internal circuits of the valve positioner 100, is generated on/the positive side of shunt regulator 4. The shunt regulator 4 may comprise one or more Zener diodes, integrated circuits, or combinations thereof with their peripheral elements.
Impedance control circuit 1 is connected to input terminals T1 and T2 and operates to adjust the impedance of variable impedance circuit 3 to control the voltage between input terminals T1 and T2 normally to an approximately constant voltage of 12V or less. The operation maintains the impedance between input terminals T1 and T2 in a low state in the DC region of the operating signal. The variable impedance circuit 3 may comprise npn transistors, pnp transistors, or field effect transistors (FET).
DCxe2x80x94DC converter 5, connected in parallel to shunt regulator 4, is used to increase the current capacity by stepping down internal power voltage V2 supplied by shunt regulator 4. Thus, DCxe2x80x94DC converter 5 supplies operating voltage V3 to current-to-pneumatic conversion module (called xe2x80x9cE/P modulexe2x80x9d) 14 which consumes high power and micro-controller 9. Since the valve positioner 100 must be operated so that its minimum operating current is 4 mA at most and normally is 3.6 mA or less because of the limitation of the input signal current, the desired current capacity is achieved by using DCxe2x80x94DC converter 5. The DCxe2x80x94DC converter 5 may comprise a voltage stepping down DCxe2x80x94DC converter, such as a charge pump type or a switching regulator type.
Current detecting or sensing element 2 and current detector 7 detect a current signal inputted to input terminals T1 and T2 and the detected signal is set to A/D converter (ADC) 8. The current detecting element 2 is a resistor and the current detector 7 is an amplifier using an operational amplifier.
Transmit-and-receive circuits 6 receive a request signal, sent from a corresponding instrument (not shown) and transmit a response signal to the corresponding instrument via digital communication. In this case, the corresponding instrument is connected to input terminals T1 and T2 via a two wire transmission line.
Micro-controller 9, which carries out digital communication with and position control to valve 16, comprises a microprocessor and peripheral circuits, such as a memory, and stores communication processing programs, such as request signals, and response signals, and control programs, such as PID control and fuzzy control. Digital to analog converter (DAC) 10 converts a digital control output signal of the micro-controller 9 to an analog signal. Driver 13 carries out amplification and impedance conversion of the analog signal, sent from DAC 10, and transmits the resulting signal to E/P module 14. Sensor interface 11 processes the signal from the position sensor 12 and sends the resulting signal to analog to digital converter (ADC) 8. ADC 8 digitizes the input current signal, sent from current detector 7, and the position signal, from valve 16, and transmits the digitized results to micro-controller 9.
The pneumatic system operates as follows. E/P module 14 converts the input drive current to a corresponding pneumatic signal and, for example, controls the air pressure of a nozzle using a torque motor. Control relay 15 amplifies the pneumatic signal and thus, for example, drives valve 16 to be in an open or closed state using the pneumatic signal of 0.2 to 1.0 [kgf/cm2]. Since the opening of valve 16 is correlated to changes of its stem position, the stem position is detected by position sensor 12.
In the FIG. 1 system, digital communication is provided between the corresponding instrument and the valve positioner by superimposing digital signals according to a predetermined protocol on a two wire transmission line that sends and receives operating signals, such as of 4 to 20 mA value. In addition, for implementing digital communication with the corresponding instrument, it is necessary to keep the impedance between the input terminals T1 and T2 at a definite high value in a communication frequency band in order to generate digital communication signals sent from the corresponding instrument between terminals T1 and T2. Accordingly, impedance control circuit 1 controls the impedance of variable impedance circuit 3 to high values of, for example, 230 ohms to 1100 ohms in the communication band.
Valve position control is provided as follows. A position signal of position sensor 12 is sent to micro-controller 9 via sensor interface 11 and ADC8, is subjected to control computation in micro-controller 9 and a resulting control output signal is sent to drive circuit 13 via DAC 10. Valve opening is controlled to a target value by driving valve 16 via the signal route of drive circuit 13xe2x86x92E/P module 14xe2x86x92control relay 15xe2x86x92valve 16.
Typical operating specifications are as follows. Minimum operating voltage between terminals: 12 V DC (between input terminals T1 and T2). Minimum operating current: 3.6 mA. That is, the digital communication function and valve position control must function within the range of 4 mA supplied to the input terminals T1 and T2. On the other hand, in the case of using a microprocessor for the micro-controller 9, even though power consumption of electronic devices is decreasing due to energy saving techniques, the current consumption for E/P modules 14 is still limited in efficiency as compared with circuits that do not use a microprocessor. However, since most E/P modules 14 are current operated devices, a problem exists in the prior art in that decreasing the current allocation to the E/P module worsens the valve response or eliminates the stability margin due to disturbances such as due to temperature.
In the microprocessor itself, the control cycle for control computation must be shortened by increasing the clock frequency to obtain stability in valve control. However, disadvantageously, another problem arises, in that current consumption in the microprocessor itself increases when the clock frequency is increased.
Hence, in order to effectively utilize the power provided to a valve positioner as an operating signal, a technique has been tried to achieve a supply current to internal circuits,including E/P modules 14, using DCxe2x80x94DC converters 5, which step down the power voltage, such as shown in FIG. 1. To realize such DCxe2x80x94DC converter 5, a charge pump type, using a capacitor or voltage stepping down switching regulator using an inductance, has been considered. However, such methods all have a further problem in that the manufacturing cost thereof increases because of the necessity to increase mounting surfaces and/or the number of components. Furthermore, disadvantageously, if the voltage stepping down switching regulator is used, adverse effects on other circuits due to switching noise, cause other problems.
U.S. Pat. No. 5,431,182 suggests another technique for effectively utilizing as an operating signal power provided to a valve positioner. This method connects two power circuits in series between the input terminals and uses one power circuit for supplying power to the digital circuits and the other power circuit for supplying power to other circuits. However, a level shift circuit to absorb differences between the two power systems is required to exchange signals between the circuits connected to the two power circuits. Thus, this prior method also has a problem in that the circuits are more complex.
The foregoing problems are also applicable to current-to-pneumatic converters.
Accordingly, as can be appreciated, the prior art needs improvement.
An object of the invention is to overcome the aforementioned and other deficiencies, problems, and disadvantages of the prior art.
Another object is to provide a valve positioner and current-to-pneumatic converter which has a reduced number of parts or components and which is simple, and wherein current allocation to the E/P module is increased.